Viscosity index improver and lubricating oil composition

ABSTRACT

The present invention aims to provide a viscosity index improver excellent in shear stability and having a low HTHS viscosity and a high viscosity index. The viscosity index improver of the present invention contains a (co)polymer (A) containing a polyolefin-based monomer as an essential monomer unit, and a base oil, wherein the absolute value of difference in solubility parameter between the (co)polymer (A) and the base oil is 0.8 to 2.0 (cal/cm 3 ) 1/2 .

This application is a 371 of PCT/JP2015/055358, filed Feb. 25, 2015.

TECHNICAL FIELD

The present invention relates to a viscosity index improver and alubricating oil composition containing the viscosity index improver.

BACKGROUND ART

A demand for lower fuel consumption of vehicles is increasing in recentyears for the purposes such as a reduction in the amount of CO₂emissions and protection of oil resources. For example, one approach tolower the fuel consumption is to reduce the viscous resistance of anengine oil by lowering its viscosity. However, lower viscosity causesproblems such as oil leakage and seizure. In cold regions, startabilityat low temperatures is required. With regard to these issues, the USSociety for Automotive Engineering (SAE) specifies the viscosity in thestandard for Engine Oil Viscosity Classification (SAE J300). For 0W-20grade oil, the high temperature high shear (HTHS) viscosity at 150° C.(ASTM D4683 or D5481) is specified to be Min. 2.6. In addition, for thesame grade oil, the low temperature viscosity at −40° C. is specified tobe 60,000 mPa·s or less with no yield stress (ASTM D4684) in order toensure startability in cold regions. To lower the fuel consumption,there is a demand for an engine oil that satisfies the above standardand has a lower HTHS viscosity in the effective temperature at 80° C. or100° C., and various viscosity index improvers have been suggested.Known examples of such viscosity index improvers include a methacrylicacid ester copolymer (Patent Literatures 1 to 4), an olefin copolymer(Patent Literature 5), and a comb copolymer (Patent Literatures 6 to 8).

However, these viscosity index improvers are insufficient in reducingthe HTHS viscosity at 80° C. when added to an engine oil composition.Such an engine oil composition is susceptible to the viscosity reductiondue to shear and exhibits a viscosity increase at low temperatures.

CITATION LIST Patent Literature

-   Patent Literature 1: JP-B 2732187-   Patent Literature 2: JP-B 2754343-   Patent Literature 3: JP-B 3831203-   Patent Literature 4: JP-B 3999307-   Patent Literature 5: JP-A 2005-200454-   Patent Literature 6: JP-B 3474918-   Patent Literature 7: JP-T 2008-546894-   Patent Literature 8: JP-T 2010-532805

SUMMARY OF INVENTION Technical Problem

The present invention aims to provide a viscosity index improver havingexcellent shear stability, a low HTHS viscosity in the effectivetemperature range, and a high viscosity index; and a lubricating oilcomposition containing the same.

Solution to Problem

As a result of intensive studies, the present inventors arrived at thepresent invention. Specifically, the present invention relates to aviscosity index improver containing a (co)polymer (A) containing apolyolefin-based monomer as an essential monomer unit and a base oil,wherein the absolute value of difference in solubility parameter betweenthe (co)polymer (A) and the base oil is 0.8 to 2.0 (cal/cm³)^(1/2). Thepresent invention also relates to a lubricating oil compositioncontaining the viscosity index improver and at least one additiveselected from the group consisting of a detergent, a dispersant, anantioxidant, an oiliness improver, a friction and wear modifier, anextreme pressure additive, a defoamer, a demulsifier, and a corrosioninhibitor.

Advantageous Effects of Invention

The viscosity index improver of the present invention and thelubricating oil composition containing the same provides effects such asexcellent shear stability, a low HTHS viscosity in the effectivetemperature range, and a high viscosity index.

DESCRIPTION OF EMBODIMENTS

The viscosity index improver of the present invention contains a(co)polymer (A) containing a polyolefin-based monomer as an essentialmonomer unit and a base oil, wherein the absolute value of thedifference in solubility parameter between the (co)polymer (A) and thebase oil is 0.8 to 2.0 (cal/cm³)^(1/2).

The (co)polymer (A) of the present invention is a (co)polymer containinga polyolefin-based monomer as an essential monomer unit.

The polyolefin-based monomer of the present invention is a monomerobtained by modifying a hydrocarbon polymer (described later) andreacting the modified hydrocarbon polymer with (meth)acrylic acid. Theterm “(meth)acryl” means methacryl or acryl.

Forms of modification include, for example, introduction of a hydroxylgroup into a hydrocarbon polymer and introduction of an amino group intoa hydrocarbon polymer. Specific examples include a monomer that can beobtained by esterification of a hydroxyl group-containing (co)polymerobtained by introducing a hydroxyl group into a hydrocarbon polymer[such as a hydroxyl group-containing polymer obtained by introducing ahydroxyl group into a hydrogenated polybutadiene or polybutene] with(meth)acrylic acid; and a monomer that can be obtained by amidation ofan amino group-containing (co)polymer obtained by introducing an aminogroup into a hydrocarbon polymer with (meth)acrylic acid. In terms ofHTHS viscosity and viscosity index, the number of hydroxyl groups oramino groups in the modified hydrocarbon polymers is preferably one.

The hydrocarbon polymer is a polymer containing the followinghydrocarbons (1) to (3) as monomer units. The hydrocarbon polymer may bea block polymer or random polymer. If the hydrocarbon polymer has adouble bond, the double bond may be partially or completely hydrogenatedby hydrogenation.

(1) Aliphatic unsaturated hydrocarbons [such as C2-C36 olefins (e.g.,ethylene, propylene, isobutene, 1-butene, 2-butene, pentene, heptene,diisobutylene, octene, dodecene, octadecene, triacontene, andhexatriacontene), and C2-C36 dienes (e.g., 1,2-butadiene, 1,3-butadiene,isoprene, 1,4-pentadiene, 1,5-hexadiene, and 1,7-octadiene)]

(2) Alicyclic unsaturated hydrocarbons [e.g., cyclohexene,(di)cyclopentadiene, pinene, limonene, indene, vinylcyclohexene, andethylidenebicycloheptene]

(3) Aromatic group-containing unsaturated hydrocarbons (e.g., styrene,α-methylstyrene, vinyltoluene, 2,4-dimethylstyrene, ethylstyrene,isopropylstyrene, butylstyrene, phenylstyrene, cyclohexylstyrene,benzylstyrene, crotylbenzene, vinylnaphthalene, divinylbenzene,divinyltoluene, divinylxylene, and trivinylbenzene), and the like.

In terms of HTHS viscosity and viscosity index, aliphatic unsaturatedhydrocarbons are preferred among these monomers, with C2-C36 olefins andC2-C36 dienes being more preferred, C2-C16 olefins and C2-C10 dienesbeing still more preferred, and isobutene, 1-butene, 2-butene, and1,3-butadiene being particularly preferred.

In terms of shear stability and HTHS viscosity, the number averagemolecular weight (hereinafter abbreviated to Mn) of the polyolefin-basedmonomer is preferably 1,000 to 25,000, more preferably 1,500 to 20,000,particularly preferably 2,000 to 15,000, most preferably 2,500 to10,000.

The Mn of the polyolefin-based monomer and the weight average molecularweight (hereinafter abbreviated to Mw) of a (co)polymer (A) (describedlater) can be measured by gel permeation chromatography under thefollowing conditions.

<Conditions for Measurement of Mn of the Polyolefin-Based Monomer and Mwof the (Co)Polymer (A)>

-   Device: “HLC-802A” [Tosoh Corporation]-   Column: “TSK gel GMH6” [Tosoh Corporation] two columns-   Measurement temperature: 40° C.-   Sample solution: 0.5% weight solution in tetrahydrofuran-   Amount of solution injected: 200 μl-   Detector: Refractive index detector-   Standard substance: standard polystyrene (TSK standard polystyrene)    12 samples (molecular weight: 500, 1,050, 2,800, 5,970, 9,100,    18,100, 37,900, 96,400, 190,000, 355,000, 1,090,000, and 2,890,000)    [Tosoh Corporation]

In terms of HTHS viscosity and viscosity index, the polyolefin-basedmonomer is preferably a monomer (a) represented by the following formula(1):

wherein R¹ is a hydrogen atom or a methyl group; —X¹— is a grouprepresented by —O—, —O(AO)_(m)—, or —NH— in which A is a C2-C4 alkylenegroup, m is an integer of 0 to 10, each A may be the same or differentwhen m is 2 or more, and the (AO)_(m) moieties may be randomly bonded orblock-bonded; R² is a residue in which one hydrogen atom is removed froma hydrocarbon polymer containing at least one of isobutylene or1,2-butylene as an essential structural unit; and p is a number of 0 or1.

In the formula (1), R¹ is a hydrogen atom or a methyl group. In terms ofHTHS viscosity in the effective temperature range, a methyl group ispreferred between these.

In the formula (1), —X¹— is a group represented by —O—, —O(AO)_(m)—, orNH—.

A is a C2-C4 alkylene group.

Examples of the C2-C4 alkylene group include an ethylene group, a 1,2-or 1,3-propylene group, and 1,2-, 1,3-, or 1,4-butylene group.

In addition, m is an integer of 0 to 10. In terms of HTHS viscosity inthe effective temperature range, it is preferably an integer of 0 to 4,more preferably 0 to 2.

In the case where m is 2 or more, each A may be the same or different,and the (AO)_(m) moieties may be randomly bonded or block-bonded.

In terms of HTHS viscosity in the effective temperature range, the grouprepresented by —X¹— is preferably a group represented by —O— or—O(AO)_(m)—, and it is more preferably a group represented by —O— or—O(CH₂CH₂O)—.

In addition, p is a number of 0 or 1.

In the formula (1), R² is a residue in which one hydrogen atom isremoved from a hydrocarbon polymer containing at least one ofisobutylene or 1,2-butylene as an essential structural unit.

Examples of the hydrocarbon polymer containing at least one ofisobutylene or 1,2-butylene as a structural unit include a polymercontaining isobutene, 1-butene, and 2-butene as structural units, and apolymer obtained by hydrogenating the terminal double bond of a1,2-adduct of poly(1,3-butadiene).

The hydrocarbon polymer may be a block polymer or a random polymer.

The hydrocarbon polymer containing at least one of isobutylene or1,2-butylene as an essential structural unit may further contain astructural unit other than at least one of isobutylene or 1,2-butylene.Examples of such monomer units include the aliphatic unsaturatedhydrocarbons (1), the alicyclic unsaturated hydrocarbons (2), and thearomatic group-containing unsaturated hydrocarbons (3), other thanisobutene, 1-butene, and 2-butene. If the hydrocarbon polymer has adouble bond, the double bond may be partially or completely hydrogenatedby hydrogenation.

In terms of HTHS viscosity, viscosity index, and shear stability, thetotal number of at least one of isobutylene or 1,2-butylene based on thetotal number of structural units of the hydrocarbon polymer ispreferably 30 mol % or more, more preferably 40 mol % or more,particularly preferably 50 mol % or more, most preferably 60 mol % ormore.

The total number of isobutylene and 1,2-butylene based on the totalnumber of structural units of the hydrocarbon polymer can be determinedby analyzing the hydrocarbon polymer by 13C-nuclear magnetic resonancespectroscopy and using the following equation (1). The 13C-nuclearmagnetic resonance spectrum has a peak derived from a methyl group ofisobutylene at 30-32 ppm integral value (integral value A), and a peakderived from a branched methylene group (—CH₂—CH(CH₂CH₃)—) of1,2-butylene at 26-27 ppm integral value (integral value B). The totalnumber of isobutylene and 1,2-butylene can be determined from theintegral values of the peaks and an integral value (integral value C) ofpeaks for all carbon atoms of the hydrocarbon polymer.

$\begin{matrix}{\left\lbrack {{Math}\mspace{14mu} 1} \right\rbrack\mspace{661mu}} & \; \\{{{Total}\mspace{14mu}{content}\mspace{14mu}{of}\mspace{14mu}{isobutylene}\mspace{14mu}{and}\mspace{14mu} 1},{{2\text{-}{butylene}} = {\frac{{\left( {{Integral}\mspace{14mu}{value}\mspace{14mu} A} \right) \times 2} + {\left( {{Integral}\mspace{14mu}{value}\mspace{14mu} B} \right) \times 4}}{\left( {{Integral}\mspace{14mu}{value}\mspace{14mu} C\mspace{14mu}{of}{\mspace{11mu}\;}{peaks}\mspace{14mu}{for}\mspace{14mu}{all}\mspace{14mu}{carbon}\mspace{14mu}{atoms}} \right)} \times 100}}} & (1)\end{matrix}$

The monomer (a) represented by the formula (1) can be obtained byesterification or amidation of a hydroxyl group-containing (co)polymerobtained by introducing a hydroxyl group into a hydrocarbon polymer oran amino group-containing (co)polymer obtained by introducing an aminogroup into a hydrocarbon polymer with (meth)acrylic acid.

Specific examples of the (co)polymers (Y) (i.e., the hydroxylgroup-containing (co)polymer and the amino group-containing (co)polymer)include the following hydroxyl group-containing (co)polymers (Y1) to(Y4) and an amino group-containing (co)polymer (Y5).

Alkylene oxide adduct (Y1); products such as those obtained by adding analkylene oxide (such as ethylene oxide or propylene oxide) to ahydrocarbon polymer obtained by polymerizing any one of the aliphaticunsaturated hydrocarbons (1), the alicyclic unsaturated hydrocarbons(2), the aromatic group-containing unsaturated hydrocarbons (e.g.,C2-C36 olefins) (3), and the like in the presence of an ionicpolymerization catalyst (such as a sodium catalyst).

Product obtained by hydroboration (Y2); products such as those obtainedby hydroboration of hydrocarbon polymers (e.g., the one described inU.S. Pat. No. 4,316,973).

Maleic anhydride-ene-amino alcohol adduct (Y3); products such as thoseobtained by imidization of a reaction product obtained by an enereaction between a hydrocarbon polymer having a double bond and maleicanhydride with an amino alcohol.

Product obtained by hydroformylation and hydrogenation (Y4); productssuch as those obtained by hydroformylation of a hydrocarbon polymerhaving a double bond, followed by hydrogenation (e.g., the one describedin JP-A 63-175096).

Maleic anhydride-ene-ethylene diamine adduct (Y5); products such asthose obtained by imidization of a reaction product obtained by an enereaction between a hydrocarbon polymer having a double bond and maleicanhydride with ethylene diamine.

In terms of HTHS viscosity and viscosity index, the (co)polymers (Y1),(Y2), and (Y3) are preferred among these (co)polymers (Y), with (Y1)being more preferred.

In terms of shear stability and HTHS viscosity, the number averagemolecular weight of each of these (co)polymers (Y) (i.e., the hydroxylgroup-containing (co)polymer and the amino group-containing (co)polymer)is preferably 1,000 to 25,000, more preferably 2,000 to 20,000,particularly preferably 3,000 to 15,000, most preferably 4,000 to10,000.

In terms of low temperature viscosity of the viscosity index improverand the lubricating oil composition, the crystallization temperature ofthe (co)polymer (Y) is preferably −40° C. or lower, more preferably −50°C. or lower, particularly preferably −55° C. or lower, most preferably−60° C. or lower.

The crystallization temperature of the (co)polymer (Y) and the(co)polymer (A) (described later) can be measured using a differentialscanning calorimeter “Unix (registered trademark) DSC7” (PerkinElmer),and it is a crystallization temperature as observed while isothermallycooling a sample (5 mg) of the (co)polymer (Y) or the (co)polymer (A)from 100° C. to −80° C. at a rate of 10° C./min.

In terms of HTHS viscosity and viscosity index, the (co)polymer (A) ofthe present invention is preferably a copolymer containing a monomer (b)represented by the following formula (2) as a monomer unit:

wherein R³ is a hydrogen atom or a methyl group; —X²— is a grouprepresented by —O— or —NH—; R⁴ is a C2-C4 alkylene group; R⁵ is a C1-C8alkyl group; and q is an integer of 1 to 20 in which each R⁴ may be thesame or different when q is 2 or more, and the (R⁴O)_(q) moieties may berandomly bonded or block-bonded.

In the formula (2), R³ is a hydrogen atom or a methyl group. In terms ofviscosity index, a methyl group is preferred between these.

In the formula (2), —X²— is a group represented by —O— or —NH—. In termsof viscosity index, a group represented by —O— is preferred betweenthese.

In the formula (2), R⁴ is a C2-C4 alkylene group. Examples of the C2-C4alkylene group include groups such as ethylene, isopropylene, 1,2- or1,3-propylene, isobutylene, and 1,2-, 1,3-, or 1,4-butylene groups.

In the formula (2), q is an integer of 1 to 20. In terms of viscosityindex and low temperature viscosity, q is preferably an integer of 1 to5, more preferably 1 or 2.

When q is 2 or more, each R⁴ may be the same or different, and the(R⁴O)_(q) moieties may be randomly bonded or block-bonded.

In the formula (2), R⁵ is a C1-C8 alkyl group. Specific examples includegroups such as methyl, ethyl, n-propyl, isopropyl, n-butyl, isobutyl,t-butyl, n-heptyl, isoheptyl, n-hexyl, 2-ethylhexyl, n-pentyl, andn-octyl groups.

Preferred among these C1-C8 alkyl groups in terms of viscosity index areC1-C6 alkyl groups, with C1-C5 alkyl groups being particularlypreferred, and C4 alkyl groups being most preferred.

Specific examples of the monomer (b) include methoxypropyl(meth)acrylate, methoxybutyl (meth)acrylate, methoxyheptyl(meth)acrylate, methoxyhexyl (meth)acrylate, methoxypentyl(meth)acrylate, methoxyoctyl (meth)acrylate, ethoxyethyl (meth)acrylate,ethoxypropyl (meth)acrylate, ethoxybutyl (meth)acrylate, ethoxyheptyl(meth)acrylate, ethoxyhexyl (meth)acrylate, ethoxypentyl (meth)acrylate,ethoxyoctyl (meth)acrylate, propoxymethyl (meth)acrylate, propoxyethyl(meth)acrylate, propoxypropyl (meth)acrylate, propoxybutyl(meth)acrylate, propoxyheptyl (meth)acrylate, propoxyhexyl(meth)acrylate, propoxypentyl (meth)acrylate, propoxyoctyl(meth)acrylate, butoxymethyl (meth)acrylate, butoxyethyl (meth)acrylate,butoxypropyl (meth)acrylate, butoxybutyl (meth)acrylate, butoxyheptyl(meth)acrylate, butoxyhexyl (meth)acrylate, butoxypentyl (meth)acrylate,butoxyoctyl (meth)acrylate, and esters of (meth)acrylic acid and C1-C8alcohols with 2 to 20 moles of ethylene oxide, propylene oxide, orbutylene oxide.

In terms of viscosity index, ethoxyethyl (meth)acrylate and butoxyethyl(meth)acrylate are preferred among the examples of the monomer (b).

In terms of HTHS viscosity in the effective temperature range, the(co)polymer (A) of the present invention is preferably a copolymercontaining, as a monomer unit, at least one selected from the groupconsisting of an alkyl (meth)acrylate (c) having a C1-C4 alkyl group, analkyl (meth)acrylate (d) having a C12-C36 linear alkyl group, and analkyl (meth)acrylate (e) having a C12-C36 branched alkyl group, inaddition to the monomers (a) and (b).

Examples of the alkyl (meth)acrylate (c) having a C1-C4 alkyl groupinclude methyl (meth)acrylate, ethyl (meth)acrylate, propyl(meth)acrylate, and butyl (meth)acrylate.

In terms of HTHS viscosity and viscosity index, methyl (meth)acrylateand butyl (meth)acrylate are preferred among these examples of the alkyl(meth)acrylate (c), with butyl (meth)acrylate being more preferred.

Examples of the alkyl (meth)acrylate (d) having a C12-C36 linear alkylgroup include n-dodecyl (meth)acrylate, n-tridecyl (meth)acrylate,n-tetradecyl (meth)acrylate, n-pentadecyl (meth)acrylate, n-hexadecyl(meth)acrylate, n-octadecyl (meth)acrylate, n-icosyl (meth)acrylate,n-tetracosyl (meth)acrylate, n-triacontyl (meth)acrylate, andn-hexatriacontyl (meth)acrylate.

In terms of HTHS viscosity and viscosity index, alkyl (meth)acrylateshaving a C12-C32 linear alkyl group are preferred among the examples ofthe alkyl (meth)acrylate (d), with alkyl (meth)acrylate having a C12-C28linear alkyl group being more preferred, and alkyl (meth)acrylate havinga C12-C22 linear alkyl group being particularly preferred.

The (co)polymer (A) of the present invention may contain a monomer (e)represented by the following formula (3) as a monomer unit:

wherein R⁶ is a hydrogen atom or a methyl group; —X³— is a grouprepresented by —O— or —NH—; R⁷ is a C2-C4 alkylene group; R⁸ and R⁹ areeach independently a C4-C24 linear alkyl group; and r is an integer of 0to 20 in which each R⁷ may be the same or different when r is 2 or more,and the (R⁷O)_(r) moieties may be randomly bonded or block-bonded.

In the formula (3), R⁶ is a hydrogen atom or a methyl group. In terms ofviscosity index, a methyl group is preferred between these.

In the formula (3), —X³— is a group represented by —O— or —NH—. In termsof viscosity index, a group represented by —O— is preferred betweenthese.

In the formula (3), R⁷ is a C2-C4 alkylene group. Examples of the C2-C4alkylene group include groups such as ethylene, isopropylene, 1,2- or1,3-propylene, isobutylene, and 1,2-, 1,3-, or 1,4-butylene groups.

In the formula (3), r is an integer of 0 to 20. In terms of viscosityindex, it is preferably an integer of 0 to 5, more preferably 0 to 2.

If r is 2 or more, each R⁷ may be the same or different, and the(R⁷O)_(r) moieties may be randomly bonded or block-bonded.

In the formula (3), R⁸ and R⁹ are each independently a C4-C24 linearalkyl group. Specific examples include groups such as n-butyl, n-heptyl,n-hexyl, n-pentyl, n-octyl, n-nonyl, n-decyl, n-undecyl, n-dodecyl,n-tetradecyl, n-hexadecyl, n-octadecyl, n-eicosyl, and n-tetracosylgroups.

In terms of viscosity index, C6-C24 linear alkyl groups are preferredamong C4-C24 linear or branched alkyl groups, with C6-C20 linear orbranched alkyl groups being more preferred and C8-C16 linear or branchedalkyl groups being particularly preferred.

Specific examples of the monomer (e) include 2-octyldecyl(meth)acrylate, an ester of ethylene glycol mono-2-octylpentadecyl etherand a (meth)acrylic acid, 2-octyldodecyl (meth)acrylate,2-n-decyltetradecyl (meth)acrylate, 2-n-dodecylhexadecyl (meth)acrylate,2-tetradecyloctadecyl (meth)acrylate, 2-dodecylpentadecyl(meth)acrylate, 2-tetradecylheptadecyl (meth)acrylate,2-hexadecylheptadecyl (meth)acrylate, 2-heptadecylicosyl (meth)acrylate,2-hexadecyldocosyl (meth)acrylate, 2-eicosyldocosyl (meth)acrylate,2-tetracosylhexacosyl (meth)acrylate, and N-2-octyldecyl(meth)acrylamide.

In terms of viscosity index, alkyl (meth)acrylates having a C12-C36branched alkyl group are preferred among these examples of the monomer(e), with alkyl (meth)acrylates having a C14-C32 branched alkyl groupbeing more preferred, and alkyl (meth)acrylates having a C16-C28branched alkyl group being particularly preferred.

The monomers (b) to (e) are monomers obtained by reacting a terminalhydroxy group or an amino group of a hydrocarbon-group containingcompound with a (meth)acrylic acid but not by modifying a hydrocarbonpolymer. Thus, these monomers (b) to (e) are not polyolefin-basedmonomers. In addition, those obtained by adding 2 to 20 moles ofethylene oxide, propylene oxide, or butylene oxide to C1-C8 alcohols andthose obtained by adding 1 to 20 moles of ethylene oxide, propyleneoxide, or butylene oxide to C10-C50 branched alkyl group-containingalcohols are also not obtained by modifying hydrocarbon polymers. Thus,these monomers are not polyolefin-based monomers.

In terms of HTHS viscosity in the effective temperature range, the(co)polymer (A) of the present invention is preferably a copolymerfurther containing, as a monomer unit, at least one selected from thegroup constituting a nitrogen-containing monomer (f), a hydroxylgroup-containing monomer (g), and a phosphorus-containing monomer (h),in addition to the monomers (a) to (e).

Examples of the nitrogen-containing monomer (f) include the followingmonomers (f1) to (f4), other than the monomers (a), (b), and (e).

Amide group-containing monomer (f1):

Examples include (meth)acrylamides, monoalkyl (meth)acrylamides [thosein which one C1-C4 alkyl group is bonded to a nitrogen atom, such asN-methyl (meth)acrylamide, N-ethyl (meth)acrylamide, N-isopropyl(meth)acrylamide, N-n-butyl (meth)acrylamide, and N-isobutyl(meth)acrylamide], N—(N′-monoalkylaminoalkyl)(meth)acrylamides [thosehaving an aminoalkyl group (C2-C6) in which one C1-C4 alkyl group isbonded to a nitrogen atom, such asN—(N′-methylaminoethyl)(meth)acrylamide,N—(N′-ethylaminoethyl)(meth)acrylamide,N—(N′-isopropylamino-n-butyl)(meth)acrylamide,N—(N′-n-butylamino-n-butyl)(meth)acrylamide, andN—(N′-isobutylamino-n-butyl)(meth)acrylamide], dialkyl (meth)acrylamides[those in which two C1-C4 alkyl groups are bonded to a nitrogen atom,such as N,N-dimethyl (meth)acrylamide, N,N-diethyl (meth)acrylamide,N,N-diisopropyl (meth)acrylamide, and N,N-di-n-butyl (meth)acrylamide],N—(N′,N′-dialkylaminoalkyl)(meth)acrylamides [those having an aminoalkylgroup (C2-C6) in which two C1-C4 alkyl groups are bonded to a nitrogenatom of an aminoalkyl group, such asN—(N′,N′-dimethylaminoethyl)(meth)acrylamide,N—(N′,N′-diethylaminoethyl)(meth)acrylamide,N—(N′,N′-dimethylaminopropyl)(meth)acrylamide, andN—(N′,N′-di-n-butylaminobutyl)(meth)acrylamide]; N-vinyl carboxylic acidamides [such as N-vinylformamide, N-vinylacetamide,N-vinyl-n-isopropionic acid amide, N-vinyl-isopropionic acid amide, andN-vinylhydroxyacetamide].

Nitro group-containing monomer (f2):

Examples include 4-nitrostyrene.

Primary to tertiary amino group-containing monomer (f3):

Examples include primary amino group-containing monomers {C3-C6 alkenylamines [such as (meth)allylamine and crotylamine], and aminoalkyl(C2-C6) (meth)acrylates [such as aminoethyl (meth)acrylate]}; secondaryamino group-containing monomers {monoalkylaminoalkyl (meth)acrylates[those having an aminoalkyl group (C2-C6) in which one C1-C6 alkyl groupis bonded to a nitrogen atom, such as N-t-butylaminoethyl (meth)acrylateand N-methylaminoethyl (meth)acrylate], and C6-C12 dialkenylamines [suchas di(meth)allylamine]}; tertiary amino group-containing monomers{dialkylaminoalkyl (meth)acrylates [those having an aminoalkyl group(C2-C6) in which two C1-C6 alkyl groups are bonded to a nitrogen atom,such as N,N-dimethylaminoethyl (meth)acrylate and N,N-diethylaminoethyl(meth)acrylate], alicyclic (meth)acrylates having a nitrogen atom [suchas morpholinoethyl (meth)acrylate], aromatic monomers [such asN—(N′,N′-diphenylaminoethyl)(meth)acrylamide, N,N-dimethylaminostyrene,4-vinylpyridine, 2-vinylpyridine, N-vinylpyrrole, N-vinylpyrrolidone,and N-vinylthiopyrrolidone]}, and hydrochlorides, sulfates, phosphates,and lower alkyl (C1-C8) monocarboxylates (examples of monocarboxylicacids include acetic acid and propionic acid) of these monomers.

Nitrile group-containing monomer (f4):

Examples include (meth)acrylonitrile.

In terms of sludge dispersibility, the monomers (f1) and (f3) arepreferred among the monomers (f). More preferred areN—(N′,N′-diphenylaminoethyl)(meth)acrylamide,N—(N′,N′-dimethylaminoethyl)(meth)acrylamide,N—(N′,N′-diethylaminoethyl)(meth)acrylamide,N—(N′,N′-dimethylaminopropyl)(meth)acrylamide, N,N-dimethylaminoethyl(meth)acrylate, and N,N-diethylaminoethyl (meth)acrylate.

Hydroxyl group-containing monomer (g):

Examples include hydroxyl group-containing aromatic monomers (such asp-hydroxystyrene), hydroxyalkyl (C2-C6) (meth)acrylates [such as2-hydroxyethyl (meth)acrylate, and 2- or 3-hydroxypropyl(meth)acrylate], mono- or bis-hydroxyalkyl (C1-C4) substituted(meth)acrylamides [such as N,N-bis(hydroxymethyl)(meth)acrylamide,N,N-bis(hydroxypropyl)(meth)acrylamide, andN,N-bis(2-hydroxybutyl)(meth)acrylamide], vinyl alcohol, C3-C12 alkenols[such as (meth)allyl alcohol, crotyl alcohol, isocrotyl alcohol,1-octenol, and 1-undecenol], C4-C12 alkene monools or alkene diols [suchas 1-buten-3-ol, 2-buten-1-ol, and 2-butene-1,4-diol], hydroxyalkyl(C1-C6) alkenyl (C3-C10) ethers (such as 2-hydroxyethylpropenyl ether),and alkenyl (C3-C10) ethers or (meth)acrylates of polyhydric alcoholshaving 3 to 8 hydroxyl groups (such as glycerol, pentaerythritol,sorbitol, sorbitan, diglycerol, sugars, and sucrose) [such as(meth)allylether of sucrose].

Examples include polyoxyalkylene glycols (the carbon number of thealkylene group is C2-C4, and the polymerization degree is 2 to 50),polyoxyalkylene polyols [polyoxyalkylene ethers of the polyhydricalcohols having 3 to 8 hydroxyl groups (the carbon number of thealkylene group is C2-C4, and the polymerization degree is 2 to 100)],and mono(meth)acrylates of alkyl (C1-C4) ethers of polyoxyalkyleneglycols or polyoxyalkylene polyols [such as polyethylene glycol (Mn: 100to 300) mono(meth)acrylate, polypropylene glycol (Mn: 130 to 500)mono(meth)acrylate, methoxy polyethylene glycol (Mn: 110 to 310)(meth)acrylate, lauryl alcohol ethylene oxide adduct (2 to 30 moles)(meth)acrylate, and polyoxyethylene (Mn: 150 to 230) sorbitanmono(meth)acrylate].

Examples of the phosphorus-containing monomer (h) include the followingmonomers (h1) and (h2).

Phosphate group-containing monomer (h1):

Examples include (meth)acryloyloxyalkyl (C2-C4) phosphate esters [suchas (meth)acryloyloxyethyl phosphate and (meth)acryloyloxy isopropylphosphate] and alkenyl phosphate esters [such as vinyl phosphate, allylphosphate, propenyl phosphate, isopropenyl phosphate, butenyl phosphate,pentenyl phosphate, octenyl phosphate, decenyl phosphate, and dodecenylphosphate]. The term “(meth)acryloyloxy” means acryloyloxy ormethacryloyloxy.

Phosphono group-containing monomer (h2):

Examples include (meth)acryloyloxy alkyl (C2-C4) phosphonic acids [suchas (meth)acryloyloxyethyl phosphonic acid] and alkenyl (C2-C12)phosphonic acids [such as vinylphosphonic acid, allylphosphonic acid,and octenylphosphonic acid].

In terms of coefficient of friction with the metal surface, the monomer(h1) is preferred among the monomers (h), with (meth)acryloyloxyalkyl(C2-C4) phosphate esters being more preferred, and(meth)acryloyloxyethyl phosphate being particularly preferred.

In terms of HTHS viscosity in the effective temperature range, the(co)polymer (A) is preferably a copolymer further containing a monomer(i) having two or more unsaturated groups as a monomer unit, in additionto the monomers (a) to (h).

Examples of the monomer (i) having two or more unsaturated groupsinclude divinyl benzene, C4-C12 alkadienes (such as butadiene, isoprene,1,4-pentadiene, 1,6-heptadiene, and 1,7-octadiene), (di)cyclopentadiene,vinylcyclohexene and ethylidenebicycloheptene, limonene, ethylenedi(meth)acrylate, polyalkylene oxide glycol di(meth)acrylate,pentaerythritol triallyl ether, trimethylolpropane tri(meth)acrylate,and esters disclosed in International Publication WO 01/009242 such asan ester of an unsaturated carboxylic acid having an Mn of 500 or moreand glycol and an ester of an unsaturated alcohol and a carboxylic acid.

The (co)polymer (A) may contain the following monomers (j) to (p) asmonomer units, in addition to the monomers (a) to (i).

Aliphatic hydrocarbon-based monomer (j):

Examples include C2-C20 alkenes (such as ethylene, propylene, butene,isobutylene, pentene, heptene, diisobutylene, octene, dodecene, andoctadecene).

Alicyclic hydrocarbon-based monomer (k):

Examples include cyclopentene, cyclohexene, cycloheptene, cyclooctene,and pinene.

Aromatic hydrocarbon-based monomer (l):

Examples include styrene, α-methylstyrene, vinyl toluene,2,4-dimethylstyrene, 4-ethylstyrene, 4-isopropylstyrene, 4-butylstyrene,4-phenylstyrene, 4-cyclohexylstyrene, 4-benzylstyrene, 4-crotylbenzene,indene, and 2-vinylnaphthalene.

Vinyl esters, vinyl ethers, vinyl ketones (m):

Examples include vinyl esters of C2-C12 saturated fatty acids (such asvinyl acetate, vinyl propionate, vinyl butyrate, and vinyl octanoate),C1-C12 alkyl, aryl or alkoxyalkylvinyl ether (methyl vinyl ether, ethylvinyl ether, propyl vinyl ether, butyl vinyl ether, 2-ethylhexyl vinylether, phenyl vinyl ether, vinyl-2-methoxyethyl ether, andvinyl-2-butoxyethyl ether), and C1-C8 alkyl or aryl vinyl ketones (suchas methyl vinyl ketone, ethyl vinyl ketone, and phenyl vinyl ketone).

Epoxy group-containing monomer (n):

Examples include glycidyl (meth)acrylate and glycidyl (meth)allyl ether.

Halogen-containing monomer (o):

Examples include vinyl chloride, vinyl bromide, vinylidene chloride,(meth)allyl chloride, and halogenated styrene (such as dichlorostyrene).

Ester of unsaturated polycarboxylic acid (p):

Examples include alkyl, cycloalkyl, or aralkyl esters of unsaturatedpolycarboxylic acids [C1-C8 alkyl diesters (dimethyl maleate, dimethylfumarate, diethyl maleate, and dioctylmaleate) of unsaturateddicarboxylic acids (such as maleic acid, fumaric acid, and itaconicacid)].

In terms of HTHS viscosity in the effective temperature range, theamount of the monomer (a) constituting the (co)polymer (A) is preferably1 to 50% by weight, more preferably 5 to 40% by weight, particularlypreferably 8 to 40% by weight, most preferably 10 to 30% by weight,based on the weight of the (co)polymer (A).

In terms of HTHS viscosity in the effective temperature range, theamount of the monomer (b) constituting the (co)polymer (A) is preferably1 to 80% by weight, more preferably 5 to 60% by weight, particularlypreferably 10 to 35% by weight, most preferably 10 to 30% by weight,based on the weight of the (co)polymer (A).

In terms of HTHS viscosity in the effective temperature range, the totalamount of the monomers (a) and (b) constituting the (co)polymer (A) ispreferably 10% by weight or more, more preferably 15 to 90% by weight,particularly preferably 20 to 80% by weight, most preferably 20 to 50%by weight, based on the weight of the (co)polymer (A).

In terms of HTHS viscosity in the effective temperature range, theamount of the alkyl (meth)acrylate (c) constituting the (co)polymer (A)is preferably 1 to 80% by weight, more preferably 20 to 70% by weight,particularly preferably 30 to 65% by weight, based on the weight of the(co)polymer (A).

In terms of HTHS viscosity in the effective temperature range, theamount of the alkyl (meth)acrylate (d) constituting the (co)polymer (A)is preferably 1 to 40% by weight, more preferably 1 to 35% by weight,particularly preferably 2 to 30% by weight, based on the weight of the(co)polymer (A).

In terms of HTHS viscosity in the effective temperature range and lowtemperature viscosity, the amount of the monomer (e) constituting the(co)polymer (A) is preferably 0 to 40% by weight, more preferably 1 to30% by weight, particularly preferably 1 to 25% by weight, based on theweight of the (co)polymer (A).

In terms of HTHS viscosity in the effective temperature range and lowtemperature viscosity, the amount of each of the monomers (f) to (h)constituting the (co)polymer (A) is preferably 0 to 15% by weight, morepreferably 1 to 12% by weight, particularly preferably 2 to 10% byweight, based on the weight of the (co)polymer (A).

In terms of HTHS viscosity in the effective temperature range, theamount of the monomer (i) constituting the (co)polymer (A) is preferably0.01 to 200 ppm, more preferably 0.05 to 50 ppm, particularly preferably0.1 to 20 ppm, based on the weight of the (co)polymer (A).

In terms of viscosity index and low temperature viscosity, the amount ofeach of the monomers (j) to (p) constituting the (co)polymer (A) ispreferably 0 to 10% by weight, more preferably 1 to 7% by weight,particularly preferably 2 to 5% by weight, based on the weight of the(co)polymer (A).

In the present invention, the solubility parameter (hereinafterabbreviated to SP) of the (co)polymer (A) is not limited as long as theabsolute value of the difference in SP between the (co)polymer A and thebase oil is 0.8 to 2.0. For example, the solubility parameter may be 5.8to 11.5 (cal/cm³)^(1/2).

If the SP of the (co)polymer (A) is outside the above range, the(co)polymer (A) may not be sufficiently dissolved in the base oil. Forexample, even if the (co)polymer (A) was sufficiently dissolved in thebase oil, the HTHS viscosity and the viscosity index might be low.

The SP of the (co)polymer (A) and the SP of each base oil (describedlater) are values calculated by the Fedors method (described in PolymerEngineering and Science, February, 1974, Vol. 14, No. 2, pp. 147-154).

In terms of viscosity index and solubility in the base oil, the SP ofthe (co)polymer (A) is preferably 9.1 to 10.3 (cal/cm³)^(1/2), morepreferably 9.1 to 9.7 (cal/cm³)^(1/2), particularly preferably 9.1 to9.5 (cal/cm³)^(1/2), most preferably 9.1 to 9.3 (cal/cm³)^(1/2).

The SP of the (co)polymer (A) is a value determined by calculating theSP of each monomer constituting the (co)polymer (A) and averaging theSPs of these monomers based on the molar fraction of each monomer unit.

The SP of the (co)polymer (A) can be adjusted by suitably adjusting theSP and the molar fraction of the monomers to be used.

In terms of HTHS viscosity in the effective temperature range and lowtemperature viscosity, the Mw of the (co)polymer (A) is preferably 5,000to 2,000,000. A more preferred range varies depending on the applicationof the viscosity index improver and the lubricating oil composition.Table 1 shows such ranges.

TABLE 1 Application More preferred range Still more preferred rangeParticularly preferred range Engine oil  150,000 to 1,000,000   230,000to 1,000,000 300,000 to 800,000 ATF*^(,) 12,000 to 55,000 belt-CVTF**, 5,000 to 150,000 10,000 to 80,000 (most preferably gear oil, MTF***15,000 to 50,000) Traction oil 10,000 to 600,000  12,000 to 230,000 15,000 to 150,000 *Automatic transmission fluid **Belt-continuouslyvariable transmission fluid ***Manual transmission fluid

In terms of low temperature viscosity of the viscosity index improverand the lubricating oil composition, the crystallization temperature ofthe (co)polymer (A) is preferably −30° C. or lower, more preferably −40°C. or lower, particularly preferably −50° C. or lower, most preferably−60° C. or lower.

The (co)polymer (A) can be obtained by a known production method.Specific examples include a method in which the monomer is subjected tosolution-polymerization in a solvent in the presence of a polymerizationcatalyst.

Examples of the solvent include toluene, xylene, C9-C10 alkylbenzenes,methyl ethyl ketone, and mineral oils.

Examples of the polymerization catalyst include azo catalysts (such as2,2′-azobis(2-methylbutyronitrile) and2,2′-azobis(2,4-dimethylvaleronitrile)), peroxide catalysts (such asbenzoyl peroxide, cumyl peroxide, and lauryl peroxide), and redoxcatalysts (such as mixtures of benzoyl peroxide and tertiary amines). Ifnecessary, a known chain transfer agent (such as C2-C20 alkylmercaptans)can also be used.

In terms of industrialization, the polymerization temperature ispreferably 25° C. to 140° C., more preferably 50° C. to 120° C. The(co)polymer (A) can also be obtained by bulk polymerization, emulsionpolymerization, or suspension polymerization other than the solutionpolymerization.

If the (co)polymer (A) is a copolymer, it may be any of the followingtypes: a random addition polymer, an alternating copolymer, a graftcopolymer, and a block copolymer.

The viscosity index improver of the present invention may contain, inaddition to the (co)polymer (A) and the base oil, an alkyl(meth)acrylate (co)polymer (B) other than the (co)polymer (A).

The alkyl (meth)acrylate (co)polymer (B) is not limited as long as it isan alkyl (meth)acrylate (co)polymer other than the (co)polymer (A).Examples include alkyl (meth)acrylate (co)polymers having a C1-C18linear alkyl group.

Specific examples of the alkyl (meth)acrylate (co)polymer (B) includen-octadecyl methacrylate/n-dodecyl methacrylate (molar ratio:10-30/90-70) copolymer, n-tetradecyl methacrylate/n-dodecyl methacrylate(molar ratio: 10-30/90-70) copolymer, n-hexadecyl methacrylate/n-dodecylmethacrylate/methyl methacrylate (molar ratio: 20-40/55-75/0-10)copolymer, and n-dodecyl acrylate/n-dodecyl methacrylate (molar ratio:10-40/90-60) copolymer. These may be used alone or in combination of twoor more thereof.

In the case where the (co)polymers (A) and (B) are used in combination,in terms of low temperature viscosity, the amount of the (co)polymer (B)to be used is preferably 0.01 to 30% by weight, more preferably 0.01 to20% by weight, particularly preferably 0.01 to 10% by weight, based onthe weight of the (co)polymer (A).

The base oil constituting the lubricating oil composition of the presentinvention may be any type as long as the absolute value of thedifference in solubility parameter between the base oil and the(co)polymer (A) is 0.8 to 2.0 (cal/cm³)^(1/2). Examples include mineraloils (such as solvent-refined oil, paraffin oil, high viscosity indexoil containing isoparaffin, high viscosity index oil obtained byhydrogenolysis of isoparaffin, and naphthene oil), synthetic lubricatingoils [for example, hydrocarbon-based synthetic lubricating oils (such aspoly-α-olefin-based synthetic lubricating oil), and ester-basedsynthetic lubricating oils], and mixtures of these oils. In terms ofoxidation stability, mineral oils are preferred among these.

The SP of the base oil is not limited as long as the absolute value ofthe difference between in SP between the (co)polymer (A) and the baseoil is 0.8 to 2.0 (cal/cm³)^(1/2). Preferably, the SP of the base oil is7.8 to 9.5 (cal/cm³)^(1/2). If the base oil has an SP of less than 7.8(cal/cm³)^(1/2), the HTHS viscosity and the viscosity index tend to below. If the base oil has an SP of more than 9.5 (cal/cm³)^(1/2), thesolubility of the (co)polymer (A) in such a base oil may beinsufficient.

The SP of the base oil is more preferably 7.9 to 9.0 (cal/cm³)^(1/2),still more preferably 8.0 to 8.5 (cal/cm³)^(1/2), particularlypreferably 8.0 to 8.3 (cal/cm³)^(1/2), most preferably 8.3(cal/cm³)^(1/2).

The SP of the base oil can be adjusted by the type and amount of esteroil.

In terms of HTHS viscosity in the effective temperature range, thekinematic viscosity of the base oil at 100° C. (as measured inaccordance with JIS-K2283) is preferably 1 to 15 mm²/s, more preferably2 to 5 mm²/s.

In terms of HTHS viscosity in the effective temperature range, theviscosity index (as measured in accordance with JIS-K2283) of the baseoil is preferably 100 or more, more preferably 110 or more.

The cloud point (as measured in accordance with JIS-K2269) of the baseoil is preferably −5° C. or lower, more preferably −15° C. or lower. Ifthe cloud point of the base oil is in the above range, the viscosityindex improver and the lubricating oil composition will have good lowtemperature viscosity.

In terms of kinematic viscosity of the viscosity index improver, theamount of the (co)polymer (A) in the viscosity index improver of thepresent invention is preferably 1 to 30% by weight as converted into theweight of the (co)polymer (A) in the viscosity index improver, based onthe weight of the base oil.

In the case of using the lubricating oil composition as an engine oil,the lubricating oil composition preferably contains 2 to 10% by weightof the (co)polymer (A) in a base oil having a kinematic viscosity at100° C. of 4 to 10 mm²/s.

In the case of using the lubricating oil composition as a gear oil, thelubricating oil composition preferably contains 3 to 30% by weight ofthe (co)polymer (A) in a base oil having a kinematic viscosity at 100°C. of 2 to 10 mm²/s.

In the case of using the lubricating oil composition as an automatictransmission fluid (such as ATF or belt-CVTF), the lubricating oilcomposition preferably contains 3 to 25% by weight of the (co)polymer(A) in a base oil having a kinematic viscosity at 100° C. of 2 to 6mm²/s.

In the case of using the lubricating oil composition as a tractionfluid, the lubricating oil composition preferably contains 0.5 to 10% byweight of the (co)polymer (A) in a base oil having a kinematic viscosityat 100° C. of 1 to 5 mm²/s.

Since the viscosity index improver of the present invention contains abase oil, the viscosity index improver of the present invention itselfcan function as a lubricating oil composition depending on the mixingratio of the (co)polymer (A) to the base oil, but it is described as aviscosity index improver so as to distinguish the viscosity indeximprover from a lubricating oil composition containing at least oneadditional additive (described later).

The lubricating oil composition of the present invention contains, inaddition to the viscosity index improver of the present invention, atleast one of the following various additives. Examples of additives aredescribed below.

(1) Detergent:

Examples include basic, overbased, or neutral metal salts [such asoverbased metal salts or alkaline earth metal salts of sulfonates (suchas petroleum sulfonate, alkylbenzene sulfonate, and alkylnaphthalenesulfonate)], salicylates, phenates, naphthanates, carbonates,phosphonates, and mixtures of these detergents.

(2) Dispersant:

Examples include succinimides (bis- or mono-polybutenyl succinimides),Mannich condensates, and borates;

(3) Antioxidant:

Examples include hindered phenols and aromatic secondary amines.

(4) Oiliness improver:

Examples include long-chain fatty acids and their esters (such as oleicacid and its ester), long-chain amines and their amides (such asoleylamine and oleylamide).

(5) Friction and wear modifier:

Examples include molybdenum-based compounds and zinc-based compounds(such as molybdenum dithiophosphate, molybdenum dithiocarbamate, andzinc dialkyldithiophosphate).

(6) Extreme pressure additive:

Examples include sulfur-based compounds (mono- or disulfide, sulfoxide,and sulfur phosphide compounds), phosphide compounds, and chlorinatedcompounds (such as chlorinated paraffin).

(7) Defoamer:

Examples include silicone oils, metallic soap, fatty acid ester, andphosphate compounds.

(8) Demulsifier:

Examples include quaternary ammonium salts (such as tetraalkylammoniumsalt), sulfonated oil, and phosphates (such as phosphates ofpolyoxyethylene-containing nonionic surfactants).

(9) Corrosion inhibitor:

Examples include nitrogen-containing compounds (such as benzotriazoleand 1,3,4-thiadiazolyl-2,5-bisdialkyldithiocarbamate).

Each of these additives is referred to as a component additive, and amixture of two or more of these component additives is sometimesreferred to as a package additive.

EXAMPLES

The present invention is described in detail below with reference toexamples, but the present invention is not limited to these examples.

Production Example 1

A reaction vessel equipped with a temperature adjuster, a vacuum stirrerblade, a nitrogen inlet, and a nitrogen outlet was charged withpolybutene having an unsaturated group at an end [product name“Polybutene 10N”; NOF Corporation; Mn: 1,000] (280 parts by weight), a 1mol/L solution of tetrahydrofuran-boron•tetrahydrofuran [Wako PureChemical Industries, Ltd.] (400 parts by weight), and tetrahydrofuran(400 parts by weight), and hydroboration was carried out at 25° C. for 4hours. Subsequently, water (50 parts by weight), a 3N—NaOH aqueoussolution (50 parts by volume), and a 30% by weight hydrogen peroxide (50parts by volume) were added for oxidation. The supernatant was collectedin a separating funnel, and the temperature was raised to 50° C. Then,tetrahydrofuran was removed over 2 hours under reduced pressure (in therange of 0.027 to 0.040 MPa) at the same temperature. Thus, a hydroxylgroup-containing polymer (Y2-1) was obtained. The total number ofisobutylene and 1,2-butylene based on the total number of structuralunits of the polymer (Y2-1) was 100 mol %, and the crystallizationtemperature of the polymer (Y2-1) was −60° C. or lower.

Production Example 2

A SUS pressure-resistant reaction vessel equipped with a temperatureadjuster and a stirrer was charged with polybutene having an unsaturatedgroup at an end [product name “Polybutene 200N”; NOF Corporation; Mn:2,650] (530 parts by weight) and maleic anhydride [Wako Pure ChemicalIndustries, Ltd.] (25 parts by weight), and the temperature was raisedto 220° C. while stirring. Then, an ene reaction was carried out for 4hours at the same temperature. Subsequently, the temperature was cooledto 25° C., and 2-aminoethanol (20 parts by weight) was added. Thetemperature was raised to 130° C. while stirring. Then, an imidizationreaction was carried out for 4 hours at the same temperature. Anunreacted maleic anhydride and 2-amino alcohol were removed over 2 hoursunder reduced pressure (in the range of 0.027 to 0.040 MPa) at atemperature of 120° C. to 130° C. Thus, a hydroxyl group-containingpolymer (Y3-1) was obtained. The total number of isobutylene and1,2-butylene based on the total number of structural units of thepolymer (Y3-1) was 100 mol %, and the crystallization temperature of thepolymer (Y3-1) was −60° C. or lower.

Examples 1 to 8, Comparative Examples 1 to 6

A reaction vessel equipped with a stirrer, a heating and cooling device,a thermometer, and a nitrogen inlet tube was charged with a base oil A(SP: 8.3 (cal/cm³)^(1/2); kinematic viscosity at 100° C.: 4.2 mm²/s;viscosity index: 128) (400 parts by weight), a monomer mixture describedin Table 2 (100 parts by weight), 2,2′-azobis(2,4-dimethylvaleronitrile) (0.5 parts by weight), and2,2′-azobis(2-methylbutyronitrile) (0.2 parts by weight), and thereaction vessel was purged with nitrogen (gas-phase oxygenconcentration: 100 ppm). Subsequently, the temperature was raised to 76°C. while stirring under hermetically sealed conditions, and apolymerization reaction was carried out for 4 hours at the sametemperature. After the temperature was raised to 120° C. to 130° C., anunreacted monomer was removed over 2 hours under reduced pressure (inthe range of 0.027 to 0.040 MPa) at the same temperature. Thus,viscosity index improvers (R1) to (R8) and (S1) to (S6) containingcopolymers (A1) to (A14) (respectively) and a base oil were obtained.The SP of each of these copolymers (A1) to (A14) was calculated by themethod described above, and the Mw was measured by the method describedabove. The solubility of each copolymer in the base oil was evaluated bythe following method. Table 2 shows the results.

<Method for Evaluating the Solubility of the Copolymer in the Base Oil>

The appearance of each of the viscosity index improvers (R1) to (R8) and(S1) to (S6) was visually observed, and the solubility in the base oilwas evaluated based on the following criteria.

[Criteria]

-   Good: Uniform appearance with no insoluble fractions of the    copolymer-   Poor: Non-uniform appearance with insoluble fractions of the    copolymer

TABLE 2 Example 1 2 3 4 5 6 7 8 Viscosity index improver (R1) (R2) (R3)(R4) (R5) (R6) (R7) (R8) Copolymer (A1) (A2) (A3) (A4) (A5) (A6) (A7)(A8) (a1-1) 10 20 5 15 12 30 0 40 (a1-2) 0 0 0 0 0 0 5 0 (a1-3) 0 0 0 100 0 0 0 (b-1) 0 0 0 15 0 0 5 0 (b-2) 20 10 20 5 60 5 50 2 (c-1) 10 0 010 0 10 0 40 (c-2) 30 60 50 35 0 45 0 0 (d-1) 15 10 0 7 10 5 10 10 (d-2)5 0 0 3 8 0 0 0 (d-3) 0 0 0 0 0 2 10 5 (e-1) 0 0 0 0 10 0 0 0 (e-2) 10 025 0 0 0 10 0 (f-1) 0 0 0 0 0 3 0 0 (g-1) 0 0 0 0 0 0 10 0 (h-1) 0 0 0 00 0 0 3 Subtotal 100 100 100 100 100 100 100 100 Total amount of 30 3025 45 72 35 60 42 (A) and (b) SP of (A) 9.20 9.16 9.19 9.13 9.14 9.109.55 9.12 Solubility in base oil Good Good Good Good Good Good Good GoodAbsolute value of difference in SP 0.90 0.86 0.89 0.83 0.84 0.80 1.250.82 between (A) and base oil Crystallization temp. of (A) −60° C. −60°C. −60° C. −60° C. −60° C. −60° C. −60° C. −60° C. or lower or lower orlower or lower or lower or lower or lower or lower Mw of (A) and (H)(×10⁴) 49 45 45 46 35 32 40 46 Comparative Example 1 2 3 4 5 6 Viscosityindex improver (S1) (S2) (S3) (S4) (S5) (S6) Copolymer (A9) (A10) (A11)(A12) (A13) (A14) (a1-1) 25 15 0 25 52 0 (a1-2) 0 0 0 0 0 0 (a1-3) 0 0 00 0 0 (b-1) 0 0 0 0 10 0 (b-2) 0 10 20 0 0 0 (c-1) 0 30 20 35 0 36 (c-2)53 0 0 0 33 0 (d-1) 17 0 30 0 5 0 (d-2) 5 17 0 0 0 0 (d-3) 0 0 0 30 0 32(e-1) 0 0 30 10 0 32 (e-2) 0 0 0 0 0 0 (f-1) 0 0 0 0 0 0 (g-1) 0 28 0 00 0 (h-1) 0 0 0 0 0 0 Subtotal 100 100 100 100 100 100 Total amount of25 25 20 25 62 0 (A) and (b) SP of (A) 9.06 10.36 9.18 9.07 8.84 9.19Solubility in base oil Good Poor Good Good Good Good Absolute value ofdifference in SP 0.76 2.06 0.88 0.77 0.54 0.89 between (A) and base oilCrystallization temp. of (A) −60° C. −60° C. −50° C. −60° C. −60° C.−40° C. or lower or lower or lower or lower Mw of (A) and (H) (×10⁴) 4546 40 52 39 50

The compositions of the monomers (a) to (h) described in Table 2 are asfollows.

(Y1-1): Polymer (hydrogenated polybutadiene) having a hydroxyl group atone end (product name “Krasol HLBH-5000M”; Cray Valley; proportion of1,2-butylene: 65 mol %; hydroxyl value; 10.4 mg KOH/g) [total number ofisobutylene and 1,2-butylene based on the total number of structuralunits of the polymer (Y1-1): 65 mol %; crystallization temperature ofthe polymer (Y1-1): −60° C. or lower]

(Y2-1): Product obtained by hydroboration of polybutene having anunsaturated group at one end described in Production Example 1

(Y3-1): Maleic anhydride-ene-amino alcohol adduct of polybutene havingan unsaturated group at one end described in Production Example 2

(a1-1): Product obtained by esterification of methacrylic acid of thepolymer (Y1-1) [Mn: 5,000]

(a1-2): Product obtained by esterification of methacrylic acid of thepolymer (Y2-1) [Mn: 1,100]

(a1-3): Product obtained by esterification of methacrylic acid of thepolymer (Y3-1) [Mn: 3,000]

(b-1): Ethoxyethyl methacrylate

(b-2): Butoxyethyl methacrylate

(c-1): Methyl methacrylate

(c-2): Butyl methacrylate

(d-1): N-dodecyl methacrylate

(d-2): N-tetradecyl methacrylate

(d-3): N-hexadecyl methacrylate

(e-1): 2-N-decyltetradecyl methacrylate

(e-2): 2-N-dodecylhexadecyl methacrylate

(f-1): N,N-dimethylaminoethyl methacrylate

(g-1): 2-Hydroxyethyl methacrylate

(h-1): Methacryloyloxyethyl phosphate

Examples 9 to 16 and Comparative Examples 7 to 12 0W-20 Evaluation

A stainless steel vessel equipped with a stirrer was charged with a baseoil A (SP: 8.3 (cal/cm³)^(1/2); kinematic viscosity at 100° C.: 4.2mm²/s; viscosity index: 128) (90 parts) and a package additive (InfineumP5741) (10 parts). Subsequently, the viscosity index improvers (R1) to(R8) and (S1) to (S6) were separately added to obtain lubricating oilcompositions having a HTHS viscosity at 150° C. of 2.60±0.05 (mm²/s).Thus, lubricating oil compositions (V1) to (V8) and (W1) to (W6) wereobtained.

The lubricating oil compositions (V1) to (V8) and (W1) to (W6) weremeasured for shear stability, HTHS viscosity (100° C.), HTHS viscosity(80° C.), viscosity index, and low temperature viscosity (−40° C.) bythe following methods. Table 3 shows the results.

In Comparative Example 8 in which the viscosity index improver (S2) wasadded, it was not possible to add the viscosity index improver (S2) in asufficient amount to obtain an intended HTHS viscosity (i.e., thesolubility of the viscosity index improver (S2) was insufficient). Thus,the shear stability, HTHS viscosity (100° C.), HTHS viscosity (80° C.),viscosity index, and low temperature viscosity (−40° C.) could not bemeasured.

Examples 17 to 24 and Comparative Examples 13 to 18 0W-16 Evaluation

A stainless steel vessel equipped with a stirrer was charged with a baseoil A (SP: 8.3 (cal/cm³)^(1/2); kinematic viscosity at 100°: 4.2 mm²/s;viscosity index: 128) (90 parts) and a package additive (Infineum P5741)(10 parts). Subsequently, the viscosity index improvers (R1) to (R8) and(S1) to (S6) were added to obtain lubricating oil compositions having aHTHS viscosity at 150° C. of 2.30±0.05 (mm²/s). Thus, lubricating oilcompositions (V9) to (V16) and (W7) to (W12) were obtained.

The lubricating oil compositions (V9) to (V16) and (W7) to (W12) weremeasured for shear stability, HTHS viscosity (100° C.), HTHS viscosity(80° C.), viscosity index, and low temperature viscosity (−40° C.) bythe following methods. Table 4 shows the results.

In Comparative Example 14 in which the viscosity index improver (S2) wasadded, it was not possible to add the viscosity index improver (S2) in asufficient amount to obtain an intended HTHS viscosity (i.e., thesolubility of the viscosity index improver (S2) was insufficient). Thus,the shear stability, HTHS viscosity (100° C.), HTHS viscosity (80° C.),viscosity index, and low temperature viscosity (−40° C.) could not bemeasured.

<Methods for Measuring and Calculating the Shear Stability of theLubricating Oil Composition>

The method of ASTM D 6278 was used for measurement and the method ofASTM D 6022 was used for calculation.

<Method for Measuring the HTHS Viscosity of the Lubricating OilComposition>

The method of ASTM D 5481 was used for measurement at 80° C. and 100° C.

<Method for Calculating the Viscosity Index of the Lubricating OilComposition>

The method of ASTM D 445 was used to measure the kinematic viscosity at40° C. and 100° C., and the method of ASTM D 2270 was used forcalculation.

<Method for Measuring the Low Temperature Viscosity of the LubricatingOil Composition>

The method of JPI-5S-42-2004 was used to measure the viscosity at −40°C.

TABLE 3 Example 9 10 11 12 13 14 15 16 Viscosity index (R1) (R2) (R3)(R4) (R5) (R6) (R7) (R8) improver Lubricating oil (V1) (V2) (V3) (V4)(V5) (V6) (V7) (V8) composition Shear stability (%) 7 6 7 7 8 9 9 9 HTHSviscosity 4.73 4.75 4.85 4.83 4.93 4.93 4.91 4.91 (100° C.) (mPa · s)HTHS viscosity 7.05 7.10 7.21 7.18 7.20 7.20 7.18 7.18 (80° C.) (mPa ·s) Viscosity index 238 239 236 234 235 234 234 236 Low-temperature25,000 22,000 23,000 28,000 22,000 18,000 23,000 21,000 viscosity (−40°C.) Comparative Example 7 8 9 10 11 12 Viscosity index (S1) (S2) (S3)(S4) (S5) (S6) improver Lubricating oil (W1) (W2) (W3) (W4) (W5) (W6)composition Shear stability (%) 12 Not 20 12 6 25 HTHS viscosity 5.05evaluable 4.95 5.09 5.30 4.83 (100° C.) (mPa · s) HTHS viscosity 7.827.35 7.65 8.02 7.25 (80° C.) (mPa · s) Viscosity index 232 212 231 239210 Low-temperature 35,000 20,000 22,000 18,000 25,000 viscosity (−40°C.)

TABLE 4 Example 17 18 19 20 21 22 23 24 Viscosity index (R1) (R2) (R3)(R4) (R5) (R6) (R7) (R8) improver Lubricating oil (V9) (V10) (V11) (V12)(V13) (V14) (V15) (V16) composition Shear stability (%) 6 5 6 6 7 8 8 8HTHS viscosity 4.46 4.48 4.57 4.55 4.65 4.65 4.63 4.63 (100° C.) (mPa ·s) HTHS viscosity 6.85 6.89 7.00 6.97 6.99 6.99 6.97 6.97 (80° C.) (mPa· s) Viscosity index 238 239 236 234 235 234 234 236 Low-temperature22,000 18,500 21,000 25,000 20,000 18,000 22,000 20,000 viscosity (−40°C.) Comparative Example 13 14 15 16 17 18 Viscosity index (S1) (S2) (S3)(S4) (S5) (S6) improver Lubricating oil (W7) (W8) (W9) (W10) (W11) (W12)composition Shear stability (%) 11 Not 18 10 5 23 HTHS viscosity 4.76evaluable 4.95 5.09 5.30 4.83 (100° C.) (mPa · s) HTHS viscosity 7.597.57 7.88 8.26 7.47 (80° C.) (mPa · s) Viscosity index 232 212 231 239210 Low-temperature 32,000 18,000 20,000 18,000 25,000 viscosity (−40°C.)

As shown in the results of Table 3 and Table 4, the lubricating oilcompositions (Examples 9 to 16 and Examples 17 to 24) containing theviscosity index improvers of the present invention are excellent in thatthese compositions have excellent shear stability, a low HTHS viscosityin the effective temperature range, and a high viscosity index. Incontrast, the results show that the lubricating oil compositions inComparative Examples 7 to 12 and Comparative Examples 13 to 18 are poorin at least one of shear stability, HTHS viscosity in the effectivetemperature range, and viscosity index. In addition, according to theresults of Comparative Example 9, Comparative Example 12, ComparativeExample 15, and Comparative Example 18, the shear stability is poor inthe lubricating oil compositions obtained by using a viscosity indeximprover containing the copolymer (A) not containing a polyolefin-basedmonomer as an essential structural unit, even though the absolute valueof difference in solubility parameter between the (co)polymer (A) andthe base oil is 0.8 to 2.0 in these lubricating oil compositions.

INDUSTRIAL APPLICABILITY

The viscosity index improver of the present invention and thelubricating oil compositions containing the same are suitable aslubricating oils for driving system (such as MTF, differential gear oil,ATF, and belt-CVTF), hydraulic oils (such as hydraulic oil for machines,power steering oil, and shock absorber oil), engine oils (such as oilsfor gasoline and diesel engines), and traction fluids.

The invention claimed is:
 1. A viscosity index improver comprising: a(co)polymer (A) comprising: (a) a polyolefin-based monomer unit (a)represented by the following formula (1):

wherein R¹ is a hydrogen atom or a methyl group; —X¹— is a grouprepresented by —O—, —O(AO)_(m)—, or —NH— in which A is a C2-C4 alkylenegroup, m is an integer of 0 to 10 each A may be the same or differentwhen m is 2 or more, and the (AO)_(m) moieties may be randomly bonded orblock-bonded; R² is a residue in which one hydrogen atom is removed froma hydrocarbon polymer containing at least one of isobutylene or1,2-butylene as an essential structural unit; and p is a number of 0 or1, and (b) a monomer unit (b) represented by the following formula (2):

wherein R³ is a hydrogen atom or a methyl group; —X²— is a grouprepresented by —O— or —NH—; R⁴ is a C2-C4 alkylene group; R⁵ is a C1-C8alkyl group; and q is an integer of 1 to 20 in which each R⁴ may be thesame or different when q is 2 or more, and the (R⁴O)_(q) moieties may berandomly bonded or block-bonded; and a base oil, wherein the absolutevalue of the difference in solubility parameter between the (co)polymer(A) and the base oil is 0.8 to 2.0 (cal/cm³)^(1/2).
 2. The viscosityindex improver according to claim 1, wherein the solubility parameter ofthe base oil is 7.8 to 9.5 (cal/cm³)^(1/2).
 3. The viscosity indeximprover according to claim 1, wherein the hydrocarbon polymercontaining at least one of isobutylene or 1,2-butylene as an essentialstructural unit in the formula (1) is a polymer in which the totalnumber of isobutylene and 1,2-butylene based on the total number ofstructural units is 30 mol % or more.
 4. The viscosity index improveraccording to claim 1, wherein the (co)polymer (A) is the copolymercontaining, as monomer units, 1 to 50% by weight of the monomer (a) and1 to 80% by weight of the monomer (b) based on the weight of the(co)polymer (A), with the total amount of the monomers (a) and (b) being10% by weight or more.
 5. The viscosity index improver according toclaim 1, wherein the (co)polymer (A) is the copolymer furthercontaining, as a monomer unit, at least one of an alkyl (meth)acrylate(c) having a C1-C4 alkyl group or an alkyl (meth)acrylate (d) having aC12-C36 linear alkyl group.
 6. The viscosity index improver according toclaim 5, wherein the (co)polymer (A) is the copolymer further containinga monomer (e) represented by the following formula (3) as a monomerunit:

wherein R⁶ is a hydrogen atom or a methyl group; —X³— is a grouprepresented by —O— or —NH—; R⁷ is a C2-C4 alkylene group; R⁸ and R⁹ areeach independently a C4-C24 linear alkyl group; and r is an integer of 0to 20 in which each R⁷ may be the same or different when r is 2 or more,and the (R⁷O)_(r) moieties may be randomly bonded or block-bonded. 7.The viscosity index improver according to claim 6, wherein the(co)polymer (A) is the copolymer containing, as monomer units, 5 to 40%by weight of the monomer (a), 5 to 60% by weight of the monomer (b), 1to 80% by weight of the alkyl (meth)acrylate (c), 1 to 40% by weight ofthe alkyl (meth)acrylate (d), and 1 to 30% by weight of the monomer (e),based on the weight of the (co)polymer (A).
 8. The viscosity indeximprover according to claim 1, wherein the (co)polymer (A) is acopolymer further comprising, as a monomer unit, at least one selectedfrom the group consisting of a nitrogen-containing monomer (f), ahydroxyl group-containing monomer (g), and a phosphorus-containingmonomer (h).
 9. The viscosity index improver according to claim 1,wherein the (co)polymer (A) has a weight average molecular weight of5,000 to 2,000,000.
 10. The viscosity index improver according to claim1, further comprising 0.01 to 30% by weight of a (co)polymer (B) otherthan the (co)polymer (A), based on the weight of the (co)polymer (A).11. The viscosity index improver according to claim 1, wherein the baseoil has a kinematic viscosity at 100° C. of 1 to 15 mm²/s, and the baseoil has a viscosity index of 100 or more.
 12. A lubricating oilcomposition comprising: the viscosity index improver according to claim1; and at least one additive selected from the group consisting of adetergent, a dispersant, an antioxidant, an oiliness improver, afriction and wear modifier, an extreme pressure additive, a defoamer, ademulsifier, and a corrosion inhibitor.